1. Field of the Invention
The present invention relates to a coordinate input apparatus which is capable of reading with high accuracy the position designated on the input surface.
2. Description of the Prior Art
As an example of conventional coordinate input devices for reading with high accuracy, the position designated on a surface, the device disclosed in Japanese Patent Publication No. 13074 published 3-27-84 will be cited. This coordinate input apparatus is composed of a tablet on which a plurality of parallel scanning lines are embedded, and a coordinate designator having an exciting coil which produces alternating field, the coordinate designator being provided in proximity to the scanning lines. Scanning signals in the form of, for example, a rectangular pulse are subsequently applied to the scanning lines, whereby induction signals which assume the state of being placed on the rectangular pulse scanning signals are subsequently detected. From the detected induction signal having the maximum value the corresponding rectangular pulse scanning signal is obtained, and the scanning line which has delivered this signal is determined to be the coordinate value of the scanning line closest to the coordinate designator. Then, from the maximum value of the detected induction signal and the magnitude of a given induction signal adjacent to the detected induction signal, the distance between the scanning line closest to the coordinate designator and the coordinate designator (coordinate value between the scanning lines) is determined. The sum of the coordinate value of the closest scanning line and the distance between the closest scanning line and the coordinate designator is calculated, and thus the coordinate value of the input coordinate designator is determined.
Another example of the prior art is disclosed in the specification of Japanese Patent Application No. 117761/1985 filed by the same applicant. In this coordinate input apparatus, at least two conductors are selected from a plurality of conductors embedded in a tablet, scanning signals of the same phase are simultaneously supplied to the selected conductors, the signals produced by the scanning signals which have been supplied to the conductors are detected by a coordinate designator, whether or not the polarity of these signals is inverted is judged, the signal levels before and after the inversion of the polarity of the signals are detected, and the position which the coordinate designator points is calculated and detected from the position at which the inversion of the polarity is judged and the signal levels.
In the first prior art example, it is necessary to obtain the maximum value of the detected induction signal, and for this purpose, A/D conversion is required at every loop, resulting in a long scanning time.
Furthermore, since the magnetic field excited by the coordinate designator is detected on the scanning line side of the input surface, the detecting portion exists over the entire surface of the input surface, whereby the detecting portion is susceptible to external noise and involves a danger of occurrence of errors.
On the other hand, in the second prior art example, since it is necessary to supply scanning signals of the same phase to at least two selected conductors, a driver of an independent constant-current amplitude is required for each conductor, and further two transistors, two resistors, one precision resistor and one operational amplifier are required for each conductor. Accordingly, though high precision is obtained, a high cost is disadvantageously involved in providing the circuit.
Furthermore, since a multiplicity of parts are used, it is difficult to control variability of the parts during production and it takes much trouble to adjust them after assembly.
In scanning, since an inexpensive decoder IC cannot be used because its output is always delivered from one line, a shift register must be incorporated, and the use of shift register takes much time before the relevant conductor is driven.